Silica particles are applied on a large scale, for example, as catalysts, catalyst carriers, adsorbents, drying agents and the like. For most of these applications, spherical particles of uniform size with a high crushing strength are desired.
An attractive way to prepare such particles is the sol-gel method. According to this method, a silica hydrosol is prepared by mixing an aqueous solution of an alkali metal silicate with an aqueous solution of an acid. The hydrosol is converted into droplet form and the droplets are gelled to produce substantially spherical hydrogel particles. After having reduced the alkali metal content of the hydrogel particles, they are dried and optionally calcined.
Further, according to the process described in British Patent Specification No. 1,525,386 the hydrogel particles, prepared substantially as described above, are partially dried before their cation content is reduced and they are finally dried and calcined. Silica particles thus prepared show a high crushing strength.
A problem which remained to be solved was to acquire silica spheres showing not only a high crushing strength but also a narrow distribution with respect to particle size and pore volume. Although the droplets themselves have a uniform diameter, the diameter of the hydrogel particles may vary since during drying the diameter of a hydrogel particle shrinks.
It has now been found that by carefully choosing the conditions during the partial drying, silica particles can be obtained having very narrow distribution ranges of both particle size and pore volume.